Tài liệu THE EFECT OF MAKEUP AIR ON KITCHEN HOODS ppt

CKV System Performance Testing The phrase “hood capture and containment” is defined in ASTM F1704-99 Standard Test Method for the Performance of Commercial Kitchen Ventilation Systems2 a

By Richard T Swierczyna, Associate Member ASHRAE, & Paul A Sobiski, Associate Member ASHRAE

A large portion of kitchen ventilation planning is dedicated to

properly exhausting cooking effluent Appliance layout and

energy input are evaluated, hoods are located and specified, ductwork

size and routing are determined, and exhaust fans are specified to

re-move the proper volume of air Unfortunately, much less time is

usu-ally dedicated to planning how the exhausted volume of air will be

replaced, although an air balance schedule is commonly used to

indi-cate the source of the makeup air (MUA).

Overlooking MUA delivery system

de-tails can have a negative impact on the

performance of an otherwise

well-de-signed kitchen Cross drafts and high air

velocities due to improper introduction

of MUA can result in failure of the hood

to capture and contain effluent from the

appliances This effluent spillage may

in-clude convective heat, products of

com-bustion (carbon dioxide, water and

potentially carbon monoxide), and

prod-ucts from the cooking process, such as

grease vapor and particles, odors, water

vapor, and various hydrocarbon gases

project focused on how the introduction

of makeup air affects the capture and containment (C&C) performance of com-mercial food service ventilation equip-ment The investigation included combinations of hoods, appliances, cooking conditions, MUA strategies and other factors

Three hood types were tested: wall-mounted canopy, island-wall-mounted canopy, and proximity (backshelf)

Charbroilers and griddles, representing heavy-duty and medium-duty appliances respectively, were tested during idle and representative cooking conditions

The six MUA strategies included: dis-placement ventilation (base case), ceiling diffuser, front face diffuser, air curtain dif-fuser, backwall supply, and short-circuit

supply (Figure 1) Certain features of the

hoods and local makeup air devices were modified to represent designs and con-figurations found in commercial kitchen installations, but not necessarily the best

or worst designs or configurations.

Overall commercial kitchen ventila-tion issues include indoor air quality, fire prevention, safety, employee comfort and equipment first costs, energy operating costs and maintenance costs This article presents strategies that can minimize the impact that makeup air introduction has

on hood performance

To address these MUA issues, a two-year research project was sponsored by a state government energy agency1 and large utility Subsequent testing for sev-eral manufacturers augmented this pub-lic research initiative This research Reprinted by permission from ASHRAE Journal, July 2003

© 2003 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

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details can have a negative impact

on the performance of an

To determine which MUA strategy offered the most

effec-tive operation while providing full capture and containment

(C&C), the research team tested the following hypothesis:

If the MUA strategy were to have no effect on exhaust

hood performance (i.e., equivalent to the displacement

ventilation base-case condition), then it would be

pos-sible to replace 100% of the air exhausted through the

makeup air configuration being investigated, while

main-taining C&C.

It was conclusively demonstrated that each of the MUA

strat-egies and specific configurations tested compromised the

ex-haust hood’s ability to completely capture and contain the

thermal plume and/or effluents at higher makeup airflow rates)

Temperature of the locally supplied makeup air also was shown

to effect hood performance as air density impacts the dynamics

of air movement around the hood Generally, hotter MUA

tem-peratures (e.g., greater than 90°F [32°C]) will affect hood

perfor-mance more adversely than cooler air (e.g., less than 75°F [24°C])

CKV System Performance Testing

The phrase “hood capture and containment” is defined in

ASTM F1704-99 Standard Test Method for the Performance

of Commercial Kitchen Ventilation Systems2 as “the ability of

the hood to capture and contain grease-laden cooking vapors,

convective heat and other products of cooking processes.”

Capture and containment performance testing incorporated

focusing schlieren and shadowgraph visualization systems to

verify capture and containment in accordance with ASTM

F1704-99 These technologies are a major breakthrough for

visualizing thermal and effluent plumes from cooking

pro-cesses A schlieren system presents a high-contrast image of

turbulent patterns due to the different air densities within the

thermal plume, similar to the effect we see over hot pavement

With appliances at idle (ready-to-cook) condition, C&C

evalu-ation is a relatively simple and repetitive task A realistic

surro-gate was needed to produce consistent effluent during cooking

C&C evaluations Since cooking hamburgers provide peak

ef-fluent production for approximately 10 seconds during a

six-minute cooking session, cooking with hamburgers was used as

a baseline condition for cooking plume simulation

For charbroilers, the natural gas flow was increased to match the previously established cooking plume The cooking plume simulator for the gas griddle was based on spraying water onto the hot cooking surface, using a pressure regulator and timed relay valve for control, and needle valves for fine-tuning During baseline displacement ventilation C&C tests, the exhaust flow rate was reduced until spillage of the thermal plume was observed The exhaust flow rate was then increased

in fine increments until full C&C was achieved over the test condition The airflow rate at this condition is referred to as the threshold exhaust airflow rate for complete C&C These values provided a baseline case to judge the various MUA strategies Evaluating the performance degradation due to cross drafts required a repeatable and practical disturbance For this task, a pedestal-mounted fan was located diagonally from the front corner of the hood

For most of the local MUA configurations investigated, the exhaust airflow rate was set initially to the C&C rate

deter-mined in the baseline displacement MUA test The local MUA was then increased (in a balanced room condition) until the threshold of capture and containment was exceeded (i.e., spill-age observed) This MUA rate was the airflow rate reported relative to the displacement exhaust C&C rate as the maxi-mum percentage of MUA that could be supplied without im-pacting hood performance

An exception to the general procedure for local MUA C&C testing was the ceiling four-way diffuser Testing was performed with constant 1,000 cfm (472 L/s) airflow and modulating the exhaust system to the threshold C&C condition In addition to the described protocols, MUA rates were incrementally increased

to determine the marginal increase in exhaust airflow rate This procedure led to an exhaust-to-MUA ratio determination and index of MUA effect The following discussion presents research results from the viewpoint of optimizing system performance

Displacement Diffusers

Displacement ventilation was the baseline for the study be-cause it provided a uniform, nearly laminar bulk airflow This low-velocity bulk airflow has proven optimal for attaining C&C

with the lowest exhaust rate Therefore, supply-ing makeup air through displacement diffusers as illustrated at right is an effective strategy for in-troducing replacement air Unfortunately, dis-placement diffusers re-quire floor or wall space that is usually at a premium in the commercial kitchen A pos-sible solution may be remote displacement diffusers (built into a corner) to help distrib-ute the introduction of makeup air into the kitchen when transfer air is not viable

Air Curtain Supply

Most hood manufacturers recommend limiting the percentage of MUA sup-plied through an air curtain

to less than 20% of the hood’s exhaust flow At such low air velocities, an air cur-tain may enhance C&C de-pending on design details However, in the cases tested, the air curtain was the worst performing strategy at higher airflows The negative im-pact of an air curtain is clearly illustrated above

by the schlieren flow visualization recorded during a test of a wall-mounted canopy hood operating over two underfired broilers

Introducing MUA through an air curtain is a risky option An air curtain (by itself or in com-bination with another pathway) is not recom-mended, unless velocities are kept to a minimum and the designer has access to per-formance data on the specified air curtain

con-f iguration Typical air curtains are easily adjusted, which could cause cooking effluent

to spill into the kitchen by inadvertently

creat-ing higher than specified discharge velocities.

Short-Circuit Supply (Internal MUA)

Internal MUA hoods were developed as a strategy to reduce the amount of conditioned air required by an exhaust system to meet code requirements This is accomplished by intro-ducing a portion of the untempered makeup air directly into the exhaust hood reservoir In cold climates, condensation and cooking sur-face cooling become undesirable side effects

The laboratory testing demonstrated that when short circuit hoods are op-erated with excessive inter-nal MUA, they fail to capture and contain the cooking effluent, often spilling at the back of the hood (although front spill-age is observed in the figure at right) If, how-ever, the specified exhaust rate is higher than the threshold for C&C in an exhaust-only con-figuration, the short-circuit airflow rate can

be increased accordingly, creating a condi-tion of apparent benefit on a percentage ba-sis For the short circuit configuration tested, the average MUA rate that could be introduced without causing spillage was 15% of the threshold C&C exhaust rate

Front Face Supply

Supplying air through the front face of the hood

is a configuration recom-mended by many hood manufacturers In theory, air exits the front face unit horizontally into the kitchen space However, a front face discharge with louvers or perforated face can perform poorly,

if its design does not consider discharge air velocity and direction The figure above repre-sents a poorly designed perforated face supply, which negatively affected this hood’s capture performance in the same fashion as an air cur-tain or four-way diffuser

To improve front face performance, internal baffling and/or a double layer of perforated plates may be used to improve the uniformity

of airflow In addition, greater distance be-tween the lower capture edge of the hood and the bottom of the face discharge area may de-crease the tendency of the MUA supply to interfere with hood capture and containment

In general, face discharge velocities should not exceed 150 fpm (0.75 m/s) and should exit the front face in a horizontal direction

Perforated Perimeter Supply

Perforated perimeter supply is similar to a front face supply, but the air is directed

down-Figure 1: Types of MUA

sup-ply integrated with the hood.

Displacement diffusers

Impact of air curtain

Excessive in-ternal MUA

Poorly designed perforated front face supply

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ward (see figure at right) toward the hood

capture area This may be advantageous

un-der some conditions, since the air is directed

downward into the hood capture zone

For proper hood performance, discharge

velocities should not exceed 150 fpm (0.75

m/s) from any section of the diffuser and

the distance to lower edge of the hood

should be no less than 18 in (0.5 m) If the

air is not introduced in this manner, the system begins to act like

an air curtain An increase in the plenum discharge area lowers

the velocity for a given flow of MUA and reduces the chance of

it affecting C&C If the perforated perimeter supply is extended

along the sides of the hood as well as the front, the increased

area will permit proportionally more MUA to be supplied

Four-Way Ceiling Diffusers

Four-way diffusers located close to

kitchen exhaust hoods (see figure at

right) can have a detrimental effect on

hood performance, particularly when

the flow through the diffuser

ap-proaches its design limit

Perforated plate ceiling diffusers can

be used in the vicinity of the hood, and a greater number of

ceiling diffusers reduce air velocities for a given supply rate

To help ensure proper hood performance, air from a diffuser

within the vicinity of the hood should not be directed toward

the hood If ceiling supplied air must be directed toward a

hood, the air discharge velocity at the diffuser face should be

set at a design value such that the terminal velocity does not

exceed 50 fpm (0.25 m/s) at the edge of the hood capture area

Backwall Supply

The lab testing demonstrated that

the backwall supply can be an

effec-tive strategy for introducing MUA (see

figure at right) For the backwall

sup-ply tested with a canopy hood, the

av-erage MUA rate that could be

introduced without causing spillage

was 46% of the threshold C&C exhaust rate

To help ensure proper performance, the discharge of the

backwall supply should be at least 12 in (0.3 m) below the

cooking surfaces of the appliances to prevent the relatively

high velocity introduction of MUA from interfering with

gas burners and pilot lights Backwall plenums with larger

discharge areas may provide increased airflow rates as

long as discharge velocities remain below maximum

thresh-olds Ideally, the quantity of air introduced through the

backwall supply should be no more than 60% of the hood’s

exhaust flow

Other Factors that Influence Hood Performance

Hood Style Wall-mounted canopy hoods function effectively

with a lower exhaust flow rate than single-island hoods Island canopy hoods are more sensitive to MUA supply and cross drafts than wall-mounted canopy hoods Proximity hoods exhibit lower C&C exhaust rates, and in some cases, perform the same job at one-third of the exhaust rate required by a wall-mounted hood

Cross Drafts Cross drafts have a detrimental effect on all

hood/appliance combinations, and adversely affect island canopy hoods more than wall-mounted canopy hoods A fan in

a kitchen, especially pointing at the cooking area, severely de-grades hood performance and may make capture impossible Cross drafts required at least a 37% increase in exhaust flow rate and in some cases C&C could not be achieved with a 235% increase in exhaust rate Cross drafts can result from portable fans, movement in the kitchen, or an unbalanced HVAC system

Side Panels and Overhang Side (or end) panels permit a

reduced exhaust rate in most cases, as they direct the replace-ment airflow to the front of the hood The installation of side panels improved C&C performance for static conditions an av-erage of 10% to 15% and up to 35% for dynamic (cross-draft) conditions They are a relatively inexpensive way to achieve C&C performance and reduce the total exhaust rate Partial side panels are able to provide virtually the same benefit as full panels One of the greatest benefits of side panels is to mitigate the negative effect of cross drafts An increase in overhang may increase the ability to contain large volume surges from cook-ing processes that use convection and combination ovens, steam-ers and pressure frysteam-ers, although for unlisted hoods this may mean an increase in the code-required exhaust rate

MUA Strategy and C&C Exhaust Rate

What was not anticipated during the design of the study was how sensitive the C&C threshold would be to the local intro-duction of MUA Spill conditions often were observed when as little as 10% of the exhaust rate was supplied by a given MUA

strategy Figure 2 shows a generic trend for changes in exhaust

airflow rate as MUA flow rate increases for a given hood/MUA system In this generic graph, the C&C exhaust flow rate is 3,000 cfm (1400 L/s) with no locally supplied MUA For local MUA

up to 500 cfm (236 L/s), the system did not require an increase

in the exhaust rate, as represented by the horizontal part of the curve When the MUA was increased beyond the 500 cfm (236 L/s), the exhaust rate had to increase to maintain C&C For this particular hood/MUA system, every 1 cfm (0.47 L/s) increase in MUA required a 0.75 cfm (0.35 L/s) increase in exhaust rate In the better performing MUA strategies, more local MUA can be introduced without increasing the exhaust rate to maintain C&C

Conclusions

The primary recommendation to reduce the impact that lo-cally supplied MUA may have on hood performance is to

mini-Perforated peri-meter supply

Backwall supply Four-way diffusers

mize the velocity (fpm) of the makeup air as it is introduced near the hood This can be accomplished by minimizing the volume (cfm) of makeup air through any single distribution system, by maximizing the area of the diffusers through which the MUA is supplied, or by distributing through multiple pathways Makeup air that is supplied through displacement ventila-tion diffusers, perforated diffusers located in the ceiling as far

as possible from the hood, or as transfer air from the dining room generally works well if air velocities approaching the hood are less than 75 fpm (0.25 m/s) However, makeup air introduced close to an exhaust hood has the potential to inter-fere with the hood’s ability to capture and contain The chances

of makeup air affecting hood performance increases as the percentage of the locally supplied MUA (relative to the total exhaust) is increased In fact, the 80% rule-of-thumb for sizing airflow through an MUA system may be a recipe for trouble The first step to reducing the MUA requirement is to lower the design exhaust rate This can be accomplished by prudent selec-tion and applicaselec-tion of UL-listed hoods.3 The use of side and/or back panels on canopy hoods to increase effectiveness, miti-gate cross drafts and reduce heat gain is highly recommended The next step in reducing MUA flow is to take credit for outside air that must be supplied by the HVAC system to meet code requirements for ventilating the dining room Depend-ing on the architectural layout, it may be practical to transfer most of this air to the kitchen Although this may contradict past practice, the hood performance will be superior and the kitchen environment will benefit from the contribution of the conditioned dining room air

References

1 Brohard, G., et al 2003 Makeup Air Effects on Kitchen Exhaust

Hood Performance California Energy Commission, Sacramento, Calif.

2 ASTM 1999 Test Method for Performance of Commercial Kitchen

Ventilation Systems Standard F 1704-99 American Society for Testing

and Materials, West Conshohocken, Pa.

3 1999 ASHRAE Handbook—HVAC Applications Chapter 30,

Kitchen Ventilation.

Richard T Swierczyna is the lab operations manager and Paul A Sobiski is a research engineer at Architectural Energy

in Wood Dale, Ill.

6,500 5,500 4,500 3,500 2,500 1,500 500 0

0 1,000 2,000 3,000 4,000 5,000

Makeup Airflow Rate (cfm)

MUA Introduction with No Effect

on C&C C&C for Exhaust Only Condition

MUA Has More of an Effect on Hood Performance

MUA Has Less of an Effect on Hood Performance

Figure 2: Potential impact of MUA on exhaust flow rates.

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